1. Field of the Invention
The present invention relates to a lighting device comprising a light source and an optical waveguide, and a liquid crystal display device using the same.
2. Description of the Prior Art
Liquid crystal display devices have widely been used for electronic devices such as portable telephones, personal digital assistants (PDA) and the like because of thin formation, light weight, and small power consumption. Lighting devices called backlights are usually disposed in the liquid crystal display devices used for such electronic devices.
FIG. 1 is a schematic diagram showing an example of a conventional liquid crystal display device. As shown in FIG. 1, the liquid crystal display device comprises a liquid crystal panel 10, and a backlight 20 arranged on a backside of the liquid crystal panel 10.
The liquid crystal panel 10 is formed by sealing a liquid crystal 12 between two transparent substrates 11a and 11b. Polarizing plates (not shown) are arranged on both sides of the liquid crystal panel 10 in a thickness direction thereof.
The backlight 20 comprises a light emitting diode (LED) 21 which becomes a light source, an optical waveguide 22, a reflective sheet 23, and a prism sheet 24. The LED 21 is arranged on one end surface side of the optical waveguide 22. In the case of a 2-inch liquid crystal panel, three to four LED's 21 are usually used.
The optical waveguide 22 is made of a transparent resin, and wedge-shaped in section as shown in FIG. 1. The reflective sheet 23 is arranged on a backside of the optical waveguide 22, and the prism sheet 24 is arranged on a front side (liquid crystal panel 10 side).
In the liquid crystal display device configured in such a manner, light emitted from the LED 21 enters the optical waveguide 22, is reflected by the reflective sheet 23, and emitted toward the liquid crystal panel 10.
A pixel electrode is formed for each pixel in one of the two transparent substrates 11a and 11b constituting the liquid crystal panel 10, and a common electrode is formed in the other substrate to face the pixel electrode. An amount of light transmitted through the pixels can be controlled by voltage applied between the pixel electrode and the common electrode. A desired image can be then displayed by controlling a light transmission amount for each pixel.
In the liquid crystal display device, light emitted from the backlight 20 preferably uniformly illuminates an entire surface of the liquid crystal panel 10. Accordingly, the fine concave and convex portions are formed on the front side or the backside of the optical waveguide 22 to scatter the light more uniformly, or the prism sheet 24 is arranged as a light distribution control plate between the optical waveguide 22 and the liquid crystal panel 10 as shown in FIG. 1.
However, if only the LED 21 is arranged near the end surface of the optical waveguide 22 as shown in FIG. 1, uneven brightness occurs to cause a problem of reduction in quality of an image displayed in the liquid crystal display device. FIG. 2 is a plan view when the backlight 20 is seen from the liquid crystal panel 10 side. As shown in FIG. 2, a plurality of LED's 21 are usually used for the liquid crystal display device. However, if only the LED's 21 are arranged near the end surface of the optical waveguide 22, light does not reach a region between adjacent LED's 21, creating dark portions (portions indicated by A in FIG. 2), and portions of high luminance (portions indicated by B in FIG. 2) are created near the front of the LED's 21.
Various technologies have conventionally been developed to solve the aforementioned problems. For example, in Patent Document 1, Japanese Patent Laid-Open No. 2004-163886 discloses a lighting device in which concave lenses are arranged between an optical waveguide and each of light sources. In this lighting device, the light emitted from the light sources is refracted by the concave lenses. Thus, occurrence of uneven brightness can be avoided. Moreover, in Patent Document 2, Japanese Patent Laid-Open No. 2002-357823, as shown in FIG. 3A, an optical waveguide 26 having a semicircular notch formed in a portion corresponding to an LED 21 is described. In this optical waveguide 26, light emitted from the LED 21 is refracted by the notch. Thus, the light reaches a region between adjacent LED's 21 to prevent uneven brightness.
Further, in Patent Document 3, Japanese Patent Laid-Open No. 2003-331628, as shown in FIG. 3B, formation of many prisms (triangular concave and convex portions) 27a in an entire end surface of an LED arranged side of an optical waveguide 27 is described. In this optical waveguide 27, light emitted from an LED 21 is refracted by the prism 27a. Thus, the light reaches a region between adjacent LED's 21 to suppress uneven brightness.
Furthermore, as shown in FIG. 3C, there is an optical waveguide 28 having fine concave and convex portions formed in an end surface of its LED arranged side. Such fine concave and convex portions are formed by blast processing with a mold block. In the optical waveguide 28, light emitted from an LED 21 is scattered by the fine concave and convex portions when it enters the optical waveguide, and reaches a region between adjacent LED's 21 to prevent uneven brightness.
FIG. 4 is a schematic diagram showing a method of manufacturing the optical waveguide 28 shown in FIG. 3C. As shown in FIG. 4, the mold block 41 is subjected to a blast processing. To be more specific, concave and convex portions are formed on a surface by spraying sands (abrasive grains) injected through a nozzle 42 to a mold block 41. At this time, concave and convex patterns can be changed by adjusting a material, a particle diameter, an injecting speed, an injecting amount, an injecting angle or the like of sand. Next, the optical waveguide 28 is molded by using the mold block 41. Subsequently, an LED, a reflective sheet, a prism sheet and the like are mounted to the optical waveguide 28 to constitute a backlight, and optical characteristic (uniformity) is evaluated by lighting the LED. Then, if desired optical characteristic is not obtained, blast processing is performed again by changing conditions.
However, the conventional lighting devices using the optical waveguides shown in FIGS. 3A to 3C have the following problems. That is, in the lighting device using the optical waveguide 26 shown in FIG. 3A, the LED 21 and the semicircular notch must be fairly accurately aligned with each other. The device lacks versatility because the number and positions of LED's 21 are determined by the notch of the optical waveguide 26. Thus, it is not easy to deal with changes in panel size.
In the lighting device using the optical waveguide 27 shown in FIG. 3B, the LED 21 and the prism 27a must be fairly accurately aligned with each other. A certain distance is necessary between the LED 21 and the optical waveguide 27 to effectively use the prism 27a. Thus, leakage light not entering the optical waveguide 27 increases to reduce light usage efficiency, causing a problem of reduction in an amount of light emitted to the liquid crystal panel side.
In the optical waveguide 28 shown in FIG. 3C, fairly accurate alignment is not necessary because of the formation of the fine concave and convex portions in the entire end surface on the LED arranged side. However, the concave and convex portions exhibiting desired characteristics must be formed by repeating the blast processing for the mold block 41, the molding and the optical evaluation. Thus, there is a drawback that the mold manufacturing takes time. In the case of a portable telephone, many molds are necessary because there are many manufacturing steps. However, reproducibility of concave and convex portions by the blast processing is low, causing a problem of nonuniform mold qualities, which in turn causes an increase in manufacturing cost.
Moreover, even the lighting device disclosed in Japanese Patent Laid-Open No. 2004-163886 involves a complicated procedure in which the light sources are aligned with the lenses with high precision. In addition, a large space needs to be secured between the optical waveguide and each of the light sources, leading to a decrease of light usage efficiency.